Electrochemical cells

ABSTRACT

A multi-bipolar electrode type of electrochemical cell which is adapted for operation with flowing electrolyte, comprises a plurality of bipolar electrode units, which can be, for example, rods, such as hollow rods, preferably horizontal, in vertical column, or groups of bipolar electrode units, which can be of geometrical shape, such as cylinders or rings, such as Lessing rings, arranged in horizontal layers one below the other separated by openwork, electrically insulating, spacers, such as nylon net. It is arranged that the electrolyte flows to bridge the spaces between adjacent electrode units, or groups of electrode units, but that any electrolyte in contact with the remainder of each electrode unit surface is kept to a very thin film. In this way electronic continuity through the cell is maintained by means of the electrolyte bridges and each electrode unit functions as a bipolar electrode.

United States Patent 1191 Fleischmann et al.

14 1 Dec. 24, 1974 ELECTROCHEMICAL CELLS National Research Development Corporation, London, England 221 Filed: Dec. 26, 1972 21 Appl. No.: 318,463

[73] Assignee:

3,335,078 8/1967 Mehl 204/268 3,451,914 6/1969 Colman 204/268 3,761,383 9/1973 Backhurst et a1 204/268 Primary Examiner-F. C. Edmundson Attorney, Agent, or FirmKemon, Palmer & Estabrook [57] ABSTRACT A multi-bipolar electrode type of electrochemical cell which is adapted for operation with flowing electrolyte, comprises a plurality of bipolar electrode units, which can be, for example, rods, such as hollow rods, preferably horizontal, in vertical column, or groups of bipolar electrode units, which can be of geometrical shape, such as cylinders or rings, such as Lessing rings,

30 FAl't'P"tDt 1 D 2:2 mglca 3, Flor. y a a 60763 7 arranged in horizontal layers one below the other SCP-r mat mam arated by openwork, electrically insulating, spacers, such as nylon net. It is arranged that the electrolyte [52] Cl fig 5 522 lfig fd flows to bridge the spaces between adjacent electrode 51 I t Cl Bolk 3/04 colb 7/06 units, or groups of electrode units, but that any elec- 136/10 13 14. trolyte in contact with the remainder of each elec- 0 care 5 3 trode unit surface is kept to a very thin film. In this way electronic continuity through the cell is main tained by means of the electrolyte bridges and each [56] uNlTE g g zfr gs giqrENTs electrode unit functions as a bipolar electrode. 1,312,756 8/1919 Stover 204/268 17 Claims, 14 Drawing Figures PATENTED [1EB24I974 SHEET 10F 9 HIIIIITT FATENTEU [153241974 SHEET H [1F 9 Ill FIG 5 Ill PATENTED DEC 2 4 I974 SHEET 5 0F 9 [1-5 M NuCI 9C M] M NuCl 9C 1-3cm(20mA) VULTS ACROSS 5 CELLS PATENTED M24197! 3, 856,652

SHEET 6 BF 9 APPARENT RUB ELECTRODE AREA, cm

SPECIFIC ELECTRODE AREA,cm'

[1 1 2 3 ROD DIAM. cm.

PATENTEI] DEBZ 4 I974 sum 9 or 9 1 ELECTROCHEMICAL CELLS This invention relates to electrochemical cells.

Levtsevich and Serguina in Gigienaii Sanitariya 33(9), 22-27, 1968 have described a packed bed cell in which a bed of magnetite particles is arranged between current-carrying electrodes in a suitable cham ber through which passes electrolyte solution. In these cells, the particles, being conductive, act to provide a number of independent bipolar units when a voltage gradient exists between the current-carrying elec trodes, and the bipolar units, in effect, provide a multiplicity of small cells throughout the bed. It has been found that in this type of cell a degree of control can be achieved by providing for isolation of at least some of the bipolar units from each other. In co-pending Pat. application Ser. No. 184,001, filed Sept. 27,1971, and now U.S. Pat. No. 3,761,383 a cell is described which comprises a bed of conducting particles at least a high proportion of which are separated from each other by non-conducting means, such as by mixing nonconducting particles with the conducting particles. The cell then comprises a large number of discrete bipolar cell units distributed throughout the bed. In one arrangement of cell in which each kind of particle, conducting and non-conducting, is of substantially the same size, or size range, it is found that approximately twice the quantity (volume) of non-conducting particles as conducting particles are necessary to ensure a high proportion of discrete bipolar units. Such cells have been found to give reasonable performances as reactors for, for example, a partial oxidation of a bromide solution which can be used to oxidise propylene to propylene oxide.

We have now found that improved characteristics can be achieved by taking even greater measures to ensure positive discreteness of the distributed bipolar units in a cell of this nature.

Additionally, the voltage required for a given bipolar cell unit to drive a particular reaction determines the voltage gradient that it is necessary to apply to the cell, while the current through the cell is determined by the conductivity through the series and parallel paths of the cell units and the electrolyte network. However, there is an upper limit to the conductivity of the electrolyte which can be used in order to achieve efficient bipolar operation with tolerable Joule heating effects. It has been found, for example, that when applied voltage gradients of to 100 volts per cm. are necessary, electrolyte concentrations as low as 10 to 10 moles per litre should be used in the known types of cell. In many cases it may be desirable or necessary, however, to use higher concentrations of electrolyte than these. We propose to enable use of higher concentrations of electrolyte by arranging for only thin if any film flow of electrolyte round the bipolar units. While, in certain circumstances, such low electrolyte concentrations as mentioned above can be used in a cell designed for thin film flow, we have found that the latter permits of use of a wider range of concentrations; this is because the thin film ensures that an electrolyte path of high resistance is formed round each bipolar unit of the cell so that current flow is preferentially via the electrode processes of the bipolar units not only making for higher current efficiency but also obviating excessive Joule heating of the electrolyte. Thus, in practice, concentrations up to 10" mol. per litre or even up to 1 mol. per litre may be used.

In accordance with the present invention, a multibipolar electrode type of cell adapted for operation with flowing electrolyte, comprises a plurality of bipolar electrode units, or groups of bipolar electrode units, said bipolar electrode units or groups of bipolar electrode units being arranged, in operation, to be electrically in series in spaced relationship from one end of the electrode system to the other, series connections being provided, in operation of the cell, between the adjacent electrode units or groups of electrode units by flowing electrolyte substantially uniformly bridging the gaps between them, comparatively thin films, if any, of electrolyte being in contact with the remainder of the surfaces of individual electrode units.

Thus, in one species of the invention, a multiplicity of substantially identical elements are arranged in layers which are supported on porous insulating supports, such as of nylon net, one next to the other and arrangements are provided whereby the electrolyte solution will flow as thin films over the element surfaces. The layers may be substantially horizontal and the electrolyte can then be made to flow from a distributed supply over the surfaces of the top layer otfelements, then over the surfaces of the elements in the lower layers in turn, the flow of electrolyte being such as to cause bridging in the general direction of flow between the elements of adjacent layers. If close-fitting insulating guides for the electrolyte are provided around each element, the layers can be arranged at an angle to the horizontal, even vertically, the space between guide and element providing the thin film flow. Obviously, however, the horizontal layer arrangement will be the less complicated and this is the preferred arrangement.

The elements in this species of the invention may take any convenient form. Thus, small metal Raschig rings, or Lessing rings, arranged with a flat end on the insulating support, are suitable but other shapes are possible andthe sides of the elements need not be circular in cross-section. The term support is used here, not only in the limited sense that the elements could be supported solely by the insulating support but also in the sense that a base support capable of supporting the whole assemlby of layers could be provided, each subsequent layer other than the bottom layer being sup ported on the insulating support by the layers beneath. Of course, it is not essential that the elements should constitute the whole of each layer and spaces between elements in each layer can be occupied by nonconducting members, such as of ceramic. Such latter members may be similar to the Raschig rings and these can assist in supporting the super-posed layers, particularly if the reactor elements aie not themselves capable of supporting the upper layers; this latter expedient would be useful, for example, if the reactor elements were of lead.

In another species of the invention, the bipolar cell unit elements are of elongated form such as of rod-, or bar-, like formation and are arranged horizontally in parallel arrays, such as one above the other in a number of substantially vertical planes; they may be arranged alternately displaced from one side to the other of the vertical plane in each array, if desired. Alternatively, the elements may be arranged substantially vertically in vertical planes or other configurations. In both cases the flow of electrolyte will be downwards, in the first,

across the surface of each element and in the second, down the surface of each element, but the flow must be as a thin film in either case, with electrolyte bridging adjacent elements in the particular configuration.

In this second species, the elongated elements may be hollow and cooling or heating fluid may be passed therethrough if desired. In the majority of cases it will be desirable that the elements are substantially straight but they are not necessarily so, consistent with the requirement that they should enable electrolyte to flow round, or along, the surface of the element. While it is not essential that the entire surface of each element should be covered with flowing electrolyte, the cell will tend to be less efficient if this is not so.

In both species, the layout of a cell will be such that gaseous materials can be passed through the cell in close proximity to the elements, for instance to enable the reactant to contact products of electrolysis at the point of their production. Another advantage to be gained from the constructions according to the invention is that, additionally, or alternatively, they provide convenient spaces for disengaging any gaseous products of the reaction that it is desired to remove from the cell. In cross-section, the elements need not be circular, nor need they be homogenous one side could be graphite and the other, for example, of copper or other metal.

It will be observed that, especially in the layered species of the invention, a very high packing fraction of bipolar units can be achieved; this in itself allows more efficient operation than in the random distribution of units of the previous proposal in co-pending Application Ser. No. 184,001, filed Sept. 27, 1971 and now US. Pat. No. 3,761,383.

In order that the invention may be more clearly understood, particular forms of cell embodying the invention and their operation will now be described by way of illustration.

FIG. 1 represents one means of carrying the invention into effect and shows a vertical cross-section of an experimental cell in which layers of substantially identical elements are arranged one above the other.

FIG. 2 shows diagrammatically a typical apparatus incorporating the cell of FIG. 1 for a specific purpose.

FIGS. 3, 4, 5, 6 and 7 represent forms of a further form of cell embodying the invention and show particular features of the cell as well as possible modifications thereof.

FIGS. 8, 9 and 10 represent still further cells embodying the invention.

FIGS. 11 and 12 are views at right angles to each other illustrating use of rod-type elements that are vertical.

FIG. 13 is a variation of the design shown in FIGS. 11 and 12.

FIG. 14 is a diagrammatic view of a multi-bipolar cell of the invention in which rod electrode units may be rotated about their longitudinal axis.

Referring to FIG. 1, the cell comprises a three-part glass vessel consisting of a main container portion 10 for the reactor 13, a bottom portion 11 which, in common with portion 10 is flanged, the respective flanges being secured together in well known liquidand gastight manner with an O ring gasket between them; the top portion 14 of the container is formed with a suitable tapered portion to seal the upper end of the main portion 10 and at the same time carries a liquid inlet tube 15 for the electrolyte feed to the cell. The lower end of tube 15 terminates in a distributor 16 which has a perforated outlet device permitting electrolyte feed to the reactor 13 to be distributed substantially uniformly over the upper region of a layer 17 of ceramic Raschig rings which are randomly packed into the layer. The effect of the layer 17 of rings is further to facilitate even distribution of the electrolyte feed to the reactor.

The reactor 13 comprises an assembly of 23 horizontal layers of 3 mm. metallic (e.g., nickel) Lessing rings, each layer extending over substantially the whole of the cross-sectional area of the container 10, each layer being isolated from the next by means of an interposed separator of nylon net 18. In the drawing, only the bottom layers of Lessing rings are indicated and the remainder of the reactor is cross-hatched for simplicity.

At the top and bottom of the reactor there are electrodes, anode l9 and cathode 20, each in the form of a flat spiral of nickel wire, diameter 0.15 cm. which has a lead-in wire attached, the positive lead-wire 21 being fed to the outside of the vessel through seal 22 and the negative lead-wire 23 through the seal 24.

Electrolyte flowing from the bottom of the reactor leaves the vessel through outlet 25 in the lower portion 11 of the vessel.

In order to permit gaseous reactants to be fed to the reactor, a gas inlet tube 26 is sealed through the wall of the portion 11. The inlet tube terminates in a glass sinter 27 which ensures even distribution of the gas over the cross-section of the cell. An outlet 28 is provided for emergent gases and vapours.

Referring now to FIG. 2, the cell 10 is assembled in conjunction with apparatus for controlling the flow of electrolyte through the cell and for monitoring inlet gas reactant flow and gas and vapour products of the reaction that takes place in the reactor. In operation of the cell, electrolyte is fed from a supply 29 by means of a pump 30 through a valve 31 and flow meter 32, to the inlet distributor 16. Electrolyte will therefore be spread over the layer 17 of Raschig rings. It will be understood of course that this layer may not be necessary if satisfactory distribution of the electrolyte can be achieved with the simple perforated distributor 16; also, it should be clear that means other than Raschig rings can be used in the region 17 for the same purpose, if desired. The objective is to enable flow of electrolyte to be adjusted to enable it to descend only as thin films over the surfaces of substantially all of the Lessing rings of the reactor. In other words, the electrolyte flow is controlled so as not, for example, to flood the reactor with electrolyte. The electrolyte will flow from the reactor to the bottom portion 11 and out of the cell through the outlet 25 which is connected through an open-ended tube 33 to an overflow 34 through which it returns to the electrolyte supply 29.

Although the Raschig rings are referred to as being of metal, they can, alternatively, comprise non-metallic cores which are coated with conducting material; they may even be of metal-coated metallic cores. These rings are arranged with their cylindrical surfaces vertical. Inlet gas flow is monitored by means of a flow meter 35 and the products of the reaction are passed through tube 28 into a condenser 36, the outlet of which leads through a vapour trap 37.

The performance of the reactor will be clear from the discussion following the description of the further form of cell shown in FIGS. 3 and 4 which are, respectively, a part-sectional elevation on the plane III-III in FIG. 4 and a sectional side view at right angles to that of FIG. 3.

This further form of cell is of fabricated construction, comprising side pieces 50 which have grooves 51 in their upper portions to receive rod elements 52. The lowermost rod elements is elongated to pass through a fitting hole in one of the side pieces 50 to form an electrical connection to the cell. The elements 52 are stacked, one above the other, being separated from each other by means ofO ring spacers mounted on alternate elements. Above the stack of elements is arranged a hollow element 54 which has a hollow extension piece 55 extending outside the cell. The element 54 is provided with one or more rows of apertures on its underside so that, when electrolyte is fed to the element 54, it emerges as a number of small jets distributed over the width of the cell. The element 54 forms the other electrode of the cell.

The cell is completed by side-plates 56 which are secured to the side pieces 50 by bolts indicated at 57; this assembly is fitted in liquidtight manner into a hollow base member 58. Electrolyte fed into the cell through inlet 55 emerges from the cell through the outlet 59 in the base member.

The elements 52 may be of graphite or metal; if desired, they can comprise a non-metallic core with an external coating of conducting material. They may even comprise a metallic core with a further conductive coating it is advantageous for certain reactions to use a titanium core with a coating of a catalyst, such as platinum. If the heat generated by a particular reaction in the cell is likely to be too great for the cell to withstand otherwise, the rods or certain of them may be of hollow form to enable cooling liquid to be passed through them. Suitable headers for the ends of the rods will be evident to those skilled in the art and need not be elaborated here.

Means, not shown, may be provided for introducing a reactant or other gas into the lower end of the cell, and outlet means will then be provided at the top for removal of gases or vapours from the cell. It is to be observed that, as in the previous example of cell shown in FIG. 1, the cell design is such that any reactant or other gases introduced into the cell can make very effective contact with the electrolyte surface as the gas moves upwards past the cell elements.

As indicated in FIG. 5, more than one column of elements may be provided in a cell of the type of this second example; the gas will move upwards between the adjacent columns and thereby will maintain the advantage of this type of construction. The gas path may be varied slightly, as indicated in FIG. 6, or, even more so, as indicated in FIG. 7.

The flow of electrolyte through a cell in accordance with the invention is probably best understood from FIGS. 5, 6 and 7. The electrolyte contacts the upper surface of the top element and flows downwards over its surface on either side and the two flow paths unite beneath the element to cause a bridge of electrolyte between that element and the succeeding element, whereafter a similar flow of electrolyte occurs over the next element, and so on. In the arrangement of FIG. 7, the bridging flow of electrolyte takes a slightly different form due to the staggering of the elements on either side ofa vertical plane in each column. Doubtless other flow forms can be established. It is apparent that the elements in the rod form of cell should be arranged to lie substantially horizontally since, otherwise, there may be too great a flow of electrolyte towards one end of the element and this may starve the following elements of electrolyte. However, further O-rings may be provided to control the flow, in particular lengthwise flow.

In the case of the reactor shown in Flg. l, the electrolyte flows down the outer and inner cylindrical surfaces of the rings which are arranged with these surfaces vertical. In this form of reactor there is an electrolyte bridge set up between the bottom edges of one layer of rings and the tops of the next layer beneath, and these bridges will penetrate the nylon niet separators. It will be appreciated that in the rod-element form of cell, gaseous reactants fed into the cell below the reactor will be in contraflow to the electrolyte. If desired, the cell may be designed to make further use of this centraflow feature, in that the upward current of gas may be used to retard the flow of the films of electrolyte over the elements. In this way, a further means ofcontrolling the film thickness may be provided, other controlling features being physical properties of the electrolyte, such as surface tension, viscosity and density, and electrolyte flow rate.

In the cases of both the rod form and the ring form of cell, it will be understood that the presence of the thin film of flowing electrolyte and the flow bridging the gap between consecutive rods or rows of rings, pro vides continuity of path for current to flow between the electrodes proper of the cell. In view of the fact that the film is thin, and possibly extremely thin, it will be seen that electrolytes of relatively high strengths may be used in these forms of cell and yet permit of compara tively high voltage drop between the said consecutive bridges of electrolyte. This means that a voltage appears on opposite sides of each rod or between the ends of each Raschig, or other, ring and in consequence the rod or ring acts as a bipolar unit cell, the current through the rod or ring being in parallel with that through the electrolyte film. The relative importance of each current path depends upon the electrolyte system and the cell design employed.

To illustrate the effectiveness of the rods as bipolar electrodes in the rod-type of cell, one of the rods in the cell can be split in two along its length and insulation placed between the two parts. In this way current flowing across the rod can be measured using an ammeter connected between the two halves. Results in such tests, using a cell of the kind described immediately below and for a total of 12 rods in an electrolyte of 3% sodium chloride, are given in the following Table:

This indicates that an appreciable proportion of the current through a cell embodying the invention can be carried through the bipolar units and that the resistance of the parallel electrolyte path is high enough to avoid excessive Joule heating of the electrolyte.

In another series of tests using the split rod" technique and 0.5M NaCl and 4M NaCl electrolytes respectively, the results shown in FIG. 8 were obtained. For the two electrolytes, the percentages of current being used for the electrode process are shown and it is clear that even with the highly conductive 4M NaCl electrolyte, over 80 percent of the current is utilised in electrolysis once sufficient voltage has been applied to allow the electrode process to proceed. For these tests, five cells were used, as indicated in FIG. 8, and the tests were repeated for three different diameters of rods, also as indicated.

FIG. 9 illustrates the results oftests to show the effect of varying the diameter of graphite rod electrodes. The effective areas were calculated from marks made on the rods to indicate the extent of surface from which gas evolution occurred during the electrolysis of brine. The upper curve for specific electrode area is for 10 rods and the lower for 6 rods. It will be observed that beyond 2.5 cm. for this particular reaction, an increase of rod diameter will give only a marginal increase in effective electrode area; this has a marked effect on the specific electrode area as also indicated by FIG. 9.

An example of use of the cell shown in FIGS. 3 and 4 is in the electrolysis of a 3% solution of sodium chloride with a continuous recycle of the electrolyte using 13 graphite rods, diameter approximately /4; inch and effective length approximately inches, in the stack forming the reactor. A flow of approximately half-litre per minute of electrolyte showed that the desired thin film flow was achieved, and for an applied voltage of 46 volts, the current through each bipolar unit (as indicated by the split rod technique) was 260 MA and the current through the film was only 50 MA.

The main reactions were:

2 NaCl 2HoH 2 NaOH C1 H C1 2NaOl-I NaCl NaClO H O Analysis of the solution for sodium hypochlorite after 5 minutes and 30 minutes yielded l/6th% and by weight solution respectively, with respective power efficiencies of 100 and 50 grams per kilowatt hour in terms of sodium hypochlorite.

In another experiment, with a similar arrangement as in the first experiment, except that the solution was not recirculated, analysis of the solution for sodium hypochlorite showed that the concentration of sodium hypochlorite in the solution leaving the cell was 0.7% by weight, the power efficiency being determined as 200 grams per kilowatt hour in terms of sodium hypochlorite.

FIG. illustrates the yield, in a rod-type unit, of sodium chlorate and sodium hypochlorite from two concentrations of brine electrolytes, in which electrolysis was carried out with one pass of the electrolyte. The influence of number of rods is shown. The current efficiency over the whole of the experiment for combined chlorate and hypochlorite was in the region of 70 to 80 percent.

Reference to FIGS. 11 and 12, which are views at right angles to each other, will indicate how the reactor of FIGS. 3 and 4 may be modified to use rod-type elements that are vertical. As shown, the vertical rods 60 are arranged close enough to each other to enable electrolyte flowing from the distributor 62 to flow not only over the surfaces of the rods but also as a bridging flow down the spaces between adjacent rods. By this means thin films of electrolyte may be caused to flow over substantially the whole of the surfaces of the rods while the electrolyte bridges 61 in the spaces between the rods serve to maintain electrical continuity between the rods at each end.

FIG. 13 illustrates a variation of the design shown in FIGS. 11 and 12. Here the vertical rods are arranged to present a series connection of two parallel sets. Thus the rods are in series, the electrolyte bridges 71 providing electrical continuity between end rods 74, and a series path exists between rods 74, 75 in the form of rods 72 and electrolyte bridges 73.

Similarly there are two series paths in parallel between end rods 75 and 76, these other paths being in the form of rods 70' and electrolyte bridges 71 and rods 72 and associated electrolyte bridges 73.

The pairs of units are each enclosed in an insulating envelope 77.

Another possible construction in the case of the horizontal rod version of cell may be to drill each rod at alternate ends so that the bridging paths of electrolyte between the rods are formed by passage of electrolyte through each hole and subsequently from one side to the other of cell in alternate directions.

Perhaps it would be as well here to emphasise that the absence of a thin film of electrolyte over the surfaces of the rods is of no consequence since it is essentially only the bridging electrolyte that is required. It is observed that the film of electrolyte, when present, serves to control the throughput of electrolyte although for reasons of its conductivity the film should be as thin as possible. The alternative, holed, construction would, of course, necessitate an attenuated flow path for the electrolyte and it may be that no advantage would accrue in general. However, it is of possible use where higher rates of throughput of electrolyte may not be crucial.

A multi-bipolar rod type of cell in accordance with an aspect of the invention may comprise means whereby at least one of the rod electrode units may be rotated about its longitudinal axis. By this means an anode becomes a cathode and a cathode becomes an anode for each half of a complete revolution. Alternatively the same effect could be achieved by reciprocal movement of the rod electrode. If, for example, the anode has become passivated by an anodic process, by this means it could be cleaned by the cathodic process. Moreover, such movement of the rods can promote mixing and/or stirring of the electrolyte and this has led to increase of current flow in mass transfer reactions.

A cell in accordance with this latter aspect of the invention is shown diagrammatically in FIG. 14, which depicts a section through the cell. In this figure test circuit and electrolyte flow systems also are indicated.

The cell comprises six rod bipolar graphite electrodes which have extension shafts 81, 82 which run in suitable bearings in the walls 87, 88 on either side of the cell so that the electrodes are insulated from each other metallically. One shaft 82 of each rod is provided with a gear wheel 83 of suitable insulating material, these gear wheels meshing with each other or with a driving pinion 84 which is actuated by an electric motor 85 which may itself have a suitable output reduction gearing. The rods may therefore be revolved at a suitable speed either continuously or intermittently, with appropriate arrangements being made for reversal of rotation if and as desired. Alternatively the driving shaft of pinion 84 may be connected to manually-operated means. Being of graphite material, the rod shafts will be selflubricated in the cell walls but other bearing material may be interposed if desired.

Electrolyte from the reservoir 86 is pumped by means of pump 89, through a heat exchanger 90 and 1 flow rate meter or indicator 91, to the cell where it is distributed substantially evenly over the width of the electrode rods by means of the distributor 92 which also acts as current conductor. The electrolyte flows as a thin film over each rod and connects adjacent rods, as explained above, with an electrolyte bridge. The electrolyte is collected by the collecting device 93 in or attached to the cell and is returned to the reservoir 86 through a sampling tap 94 which enables analysis of the products of electrolysis to be made and monitoring, if necessary. Taps 95, 96 and 97 are provided to vary the flow and/or flow pattern of the electrolyte as indicated by the arrows so that, for example, electrolyte can be pumped directly to the cell from the collector 93 so as to give the facility of being able to recycle a relatively small quantity of electrolyte through the cell. A suitable power supply 98 provides the necessary current for operation of the cell.

A temperature sensitive device 99 is provided for indication and/or control of the temperature of the electrolyte.

Considering applications of the form of cell described with reference to FIGS. 1 and 2, it is of interest to note that this form has been shown to be particularly suited to the oxidation of amines and more especially to the production of propylene oxide.

Thus, using aqueous l Molar (and even up to Molar) sodium bromide solution saturated with propylene, this solution is electrolysed in the reactor while a mixture of propylene and nitrogen in an appropriate ratio is fed to the cell so that it passes through the reactor in contraflow to the flow of the solution. The bromine formed on the anodic areas of the bipolar units reacts with hydroxyl ions formed on the cathodic areas of the units to produce hypobromite ions. Hypobromous acid reacts with propylene to produce bromhydrin which further reacts with hydroxyl ions to give propylene oxide. The propylene oxide is carried out of the cell in the gas stream and may be separated out in known manner, though some may be recirculated in the electrolyte solution.

As a test of efficiency of this latter form of cell, it is found, for example, that, using %th inch nickel Lessing rings and 10 Molar potassium hydroxide solution current efficiencies as high as 65% may be achieved and in 10 Molar potassium hydroxide solution 75% efficiency has even been achieved.

What we claim is:

l. A multi-bipolar electrode cell adapted for operation with flowing electrolyte, comprising a plurality of bipolar electrode units, or groups of bipolar electrode units, said bipolar electrode units or groups of bipolar electrode units being arranged, in operation, to be electrically in series in spaced relationship from one end of the electrode system to the other, series connections being provided, in operation of the cell, between the adjacent electrode units or groups of electrode units by flowing electrolyte substantially uniformly bridging the gaps between them, comparatively thin films ofelectrolyte being in contact with the remainder of the surfaces of individual electrode units.

2. A multi-bipolar electrode cell as claimed in claim 1, in which the bipolar electrode units, or groups of bipolar electrode units, are arranged one above the other and comprising means for controlling the supply of electrolyte to the electrode system so that the flow is substantially evenly distributed over the top of the uppermost electrode unit or group of electrode units.

3. A multi-bipolar electrode cell as claimed in claim 2, wherein the geometry of the cell permits intimate contact between reactant gas and electrolyte at said bridging paths.

4. A multi-bipolar electrode cell as claimed in claim 3, wherein arrangements are made whereby reactant gas is in contra-flow to the electrolyte.

5. A multi-bipolar electrode cell as claimed in claim 1, wherein all of the bipolar electrode units are geometrically the same.

6. A multi-bipolar electrode cell as claimed in claim 1, wherein the bipolar electrode units are rods of conducting material.

7. A multi-bipolar electrode cell as claimed in claim 6, wherein the bipolar electrode units are rods of graphite.

8. A multi-bipolar electrode cell as claimed in claim 7, wherein said electrode unit rods are arranged in staggered columns so that the said bridging paths of electrolyte pass alternately from one column to the next.

9. A multi-bipolar electrode cell as claimed in claim 6, wherein a plurality of columns of electrode unit rods are arranged side by side within the cell.

10. A multi-bipolar electrode cell as claimed in claim 6, wherein said rods are hollow.

11. A multi-bipolar electrode cell as claimed in claim 10, wherein the bipolar hollow rod. electrode system is arranged for flow of fluid through the rod for heat transfer purposes.

12. A multi-bipolar electrode cell as claimed in claim 1, in which the bipolar electrode units comprise rods of conducting material arranged vertically side-by-side in at least one row and in close but separated juxtaposition one to the next and electrolyte is caused to flow to form bridging paths between adjacent rods.

13. A multi-bipolar electrode cell as claimed in claim 12, wherein the electrolyte is arranged to flow vertically down the bridging paths.

14. A multi-bipolar electrode cell as claimed in claim 6, wherein electrolyte is introduced to each electrode unit assembly through a perforated or like distributor permitting electrolyte to flow in substantially even distribution over the width of the electrode system.

15. A multi-bipolar electrode cell as claimed in claim 1 in which each bipolar electrode unit is cylindrical and is arranged with the cylindrical surface extending vertically.

16. A multi-bipolar electrode cell as claimed in claim 15, wherein said cylindrical electrode unit is a ring member.

17. A multi-bipolar electrode cell as claimed in claim 15, wherein numbers of said cylindrical units are ar ranged in parallel rows one below the other and resting on interposed openwork insulating screens which cause said substantially uniformly distributed electrolyte flow. 

1. A multi-bipolar electrode cell adapted for operation with flowing electrolyte, comprising a plurality of bipolar electrode units, or groups of bipolar electrode units, said bipolar electrode units or groups of bipolar electrode units being arranged, in operation, to be electrically in series in spaced relationship from one end of the electrode system to the other, series connections being provided, in operation of the cell, between the adjacent electrode units or groups of electrode units by flowing electrolyte substantially uniformly bridging the gaps between them, comparatively thin films of electrolyte being in contact with the remainder of the surfaces of individual electrode units.
 2. A multi-bipolar electrode cell as claimed in claim 1, in which the bipolar electrode units, or groups of bipolar electrode units, are arranged one above the other and comprising means for controlling the supply of electrolyte to the electrode system so that the flow is substantially evenly distributed over the top of the uppermost electrode unit or group of electrode units.
 3. A multi-bipolar electrode cell as claimed in claim 2, wherein the geometry of the cell permits intimate contact between reactant gas and electrolyte at said bridging paths.
 4. A multi-bipolar electrode cell as claimed in claim 3, wherein arrangements are made whereby reactant gas is in contra-flow to the electrolyte.
 5. A multi-bipolar electrode cell as claimed in claim 1, wherein all of the bipolar electrode units are geometrically the same.
 6. A multi-bipolar electrode cell as claimed in claim 1, wherein the bipolar electrode units are rods of conducting material.
 7. A multi-bipolar electrode cell as claimed in claim 6, wherein the bipolar electrode units are rods of graphite.
 8. A multi-bipolar electrode cell as claimed in claim 7, wherein said electrode unit rods are arranged in staggered columns so that the said bridging paths of electrolyte pass alternately from one column to the next.
 9. A multi-bipolar electrode cell as claimed in claim 6, wherein a plurality of columns of electrode unit rods are arranged side by side within the cell.
 10. A multi-bipolar electrode cell as claimed in claim 6, wherein said rods are hollow.
 11. A multi-bipolar electrode cell as claimed in claim 10, wherein the bipolar hollow rod electrode system is arranged for flow of fluid through the rod for heat transfer purposes.
 12. A multi-bipolar electrode cell as claimed in claim 1, in which the bipolar electrode units comprise rods of conducting material arranged vertically side-by-side in at least one row and in close but separated juxtaposition one to the next and electrolyte is caused to flow to form bridging paths between adjacent rods.
 13. A multi-bipolar electrode cell as claimed in claim 12, wherein the electrolyte is arranged to flow vertically down the bridging paths.
 14. A multi-bipolar electrode cell as claimed in claim 6, wherein electrolyte is introduced to each electrode unit assembly througH a perforated or like distributor permitting electrolyte to flow in substantially even distribution over the width of the electrode system.
 15. A multi-bipolar electrode cell as claimed in claim 1 in which each bipolar electrode unit is cylindrical and is arranged with the cylindrical surface extending vertically.
 16. A multi-bipolar electrode cell as claimed in claim 15, wherein said cylindrical electrode unit is a ring member.
 17. A multi-bipolar electrode cell as claimed in claim 15, wherein numbers of said cylindrical units are arranged in parallel rows one below the other and resting on interposed openwork insulating screens which cause said substantially uniformly distributed electrolyte flow. 